Well sure, yes, you can do that. Except it looks really shitty! My plugin let's you style and fit the form to your theme, making it blend in seamlessly.

Will auto-populate get variables into a form based off the label name (substituting "_" for spaces). So yourwebsite.com/myform?First_Name=Bob&Last_Name=Smith will fill in "Bob" and "Smith" for the inputs preceded by the labels "First Name" and "Last Name". Try THAT with your iFrame.